The steam generators of pressurized-water nuclear reactors have a bundle of tubes, the ends of which are expanded inside a tube plate of considerable thickness. The high-pressure and high-temperature primary water of the reactor which comes into contact with the fuel assemblies of the core flows inside the tubes of the bundle in order to heat and vaporize the feed water brought into contact with the external wall of the tubes, inside the casing of the steam generator.
The tubes of the bundle lie flush with one of the sides of the plate, called the inlet side, where the primary water enters and leaves the bundle in two distinct zones, and emerge from this tube plate, so as to penetrate the bundle casing, on its other side called the outlet side.
The walls of the tubes in the bundle thus form a barrier between the primary fluid containing activated particles and the feed water to be vaporized, the steam from which is passed to the turbine associated with the nuclear reactor. Therefore, the appearance of leaks in the tubes of the bundle inside the steam generator must be avoided as far as possible, and the tubes must be repaired as quickly as possible and effectively when their walls are pierced.
Due to the temperature gradients and mechanical stresses resulting from the differences in pressure and different forms of corrosion which may appear inside the steam generator, both on the primary side and on the secondary side, during use of the steam generator, the life of which must be equal to that of the other parts of the reactor, fractures may appear in the tubes, giving rise to leaks, and repairs must be carried out during maintenance of the reactor.
Until now, one of the most commonly used methods consisted in making the defective tube inoperative by means of a plug fixed to the end of the tube located in the vicinity of the inlet side of the tube plate, in the part which receives the primary water coming from the reactor core. This known method is fairly reliable, but has the drawback that it reduces the exchange area of the steam generator, to an extent which increases with the number of bundle tubes which have to be rendered inoperative.
Methods of repairing the tubes of the bundle in a steam generator have therefore been devised, consisting in lining these tubes at the point where the defect giving rise to a loss of fluid tightness occurs. A tubular liner with an external diameter slightly less than the internal diameter of the tube to be repaired and of sufficient length to cover the defect is introduced into the tube through the inlet side of the tube plate and placed inside this tube plate and inside the tube so as to lie flush with the inlet side. This liner is then fixed by means of diametrical expansion inside the tube.
For example, the suggestion has been made to achieve this expansion by means of a hydraulic chuck, but the mechanical behavior and fluid-tightness of the repaired tube have proved insufficient.
The suggestion has also been made to improve fixing of the liner by expanding it inside the tube.
The expanding operation results in rolling of the liner inside the tube and hence in a reduction in its thickness.
The suggestion has therefore been made to braze the liner inside the tube after hydraulic or mechanical expansion of the top and bottom of the said liner. Diametrical expansion of the liner inside the tube is performed hydraulically or mechanically in a zone of this liner located above the tube plate on the secondary side. Fixing is completed by melting a brazing material introduced beforehand between the external surface of the liner and the internal surface of the tube in the zone being expanded.
However, brazing poses certain problems in particular as regards distribution and homogeneity; reliability is limited as regards fluid-tightness, and the operation is complex and made more difficult by the need to provide an integrated brazing joint, associated with a brazing flux. This is why it is generally preferred to fix the liner, after deformation of the latter, by means of welding inside the tube.
In this process, the bottom end of the liner lying flush with the inlet side of the tube plate is first fixed to the inside of the tube by means of expansion, so as to form a metallurgical bond, strengthened by a peripheral welding bead.
Upper diametrical expansion of the liner inside the tube is then performed in a second zone of this liner, above the tube plate. The mechanical fixing is completed by welding the liner inside the tube, in the region of the deformed zone, thereby achieving tightness at the same time.
Using this method, it is possible to ensure mechanical fixing and satisfactory tightness of the liner inside the tube, but the deformation is generally performed over a large area of the liner, thereby giving rise to considerable internal stresses, in particular in the crucial zones, i.e., in the zones located on either side of the deformation, which may subsequently promote corrosion, in particular stress or tensile corrosion.